Advances in field emission display technology are disclosed in U.S. Pat. No. 3,755,704, "Field Emission Cathode Structures and Devices Utilizing Such Structures," issued Aug. 28, 1973, to C. A. Spindt et al.; U.S. Pat. No. 4,940,916, "Electron Source with Micropoint Emissive Cathodes and Display Means by Cathodoluminescence Excited by Field Emission Using Said Source," issued Jul. 10, 1990 to Michel Borel et al.; U.S. Pat. No. 5,194,780, "Electron Source with Microtip Emissive Cathodes," issued Mar. 16, 1993 to Robert Meyer; and U.S. Pat. No. 5,225,820, "Microtip Trichromatic Fluorescent Screen," issued 6 Jul. 6, 1993, to Jean-Frederic Clerc. These patents are incorporated by reference into the present application.
The Clerc ('820) patent discloses a trichromatic field emission flat panel display having a first substrate, on which are arranged a matrix of conductors. The first substrate is also called the cathode plate or the emitter plate. In one direction of the matrix, conductive columns comprising the cathode electrode support the microtips. In the other direction, above the column conductors, are perforated conductive rows comprising the grid electrode. The row and column conductors are separated by an insulating layer having apertures permitting the passage of the microtips, each intersection of a row and column corresponding to a pixel.
On a second substrate, facing the first, the display has regularly spaced, parallel conductive stripes comprising the anode electrode. The second substrate is also called the anode plate. These stripes are alternately covered by a first material luminescing in red, a second material luminescing in green, and a third material luminescing in blue, the conductive stripes covered by the same luminescent material being electrically interconnected.
The Clerc patent discloses a process for addressing a trichromatic field emission flat panel display. The process consists of successively raising each set of interconnected anode stripes periodically to a first potential which is sufficient to attract the electrons emitted by the microtips of the cathode conductors corresponding to the pixels which are to be illuminated in the color of the selected anode stripes. Those anode stripes which are not being selected are set to a potential such that the electrons emitted by the microtips are repelled or have an energy level below the threshold cathodoluminescence energy level of the luminescent materials covering those unselected anodes.
Referring initially to FIG. 1, there is shown, in cross-sectional view, a portion of an illustrative prior art field emission device in which the present invention may be incorporated. This device comprises an anode plate 1 having a cathodoluminescent phosphor coating 3 facing an emitter plate 2, the phosphor coating 3 being observed from the side opposite to its excitation.
More specifically, the field emission device of FIG. 1 comprises an anode plate 1 and an electron emitter (or cathode) plate 2. A cathode portion of emitter plate 2 includes conductors 9 formed on an insulating substrate 10, an electrically resistive layer 8 which is formed on substrate 10 and overlaying the conductors 9, and a multiplicity of electrically conductive microtips 5 formed on the resistive layer 8. In this example, the conductors 9 comprise a mesh structure, and microtip emitters 5 are configured as a matrix within the mesh spacings. Microtips 5 take the shape of cones which are formed within apertures through an electrically conductive layer 6 and an insulating layer 7.
A gate electrode comprises the layer of the electrically conductive material 6 which is deposited on the insulating layer 7. The thicknesses of gate electrode layer 6 and insulating layer 7 are chosen in such a way that the apex of each microtip 5 is substantially level with the electrically conductive gate electrode layer 6. Conductive layer 6 may be in the form of a continuous layer across the surface of substrate 10. Alternatively, as described in the Borel '161 patent, it may comprise conductive bands across the surface of substrate 10.
Anode plate 1 comprises a transparent, electrically conductive film 12 deposited on a transparent planar support 13, such as glass, which is positioned facing gate electrode 6 and parallel thereto, the conductive film 12 being deposited on the surface of the glass support 13 directly facing gate electrode 6. Conductive film 12 may be in the form of a continuous layer across the surface of the glass support 13; alternatively, it may be in the form of electrically isolated stripes comprising three series of parallel conductive bands across the surface of the glass support 13, as shown in FIG. 1 and as taught in U.S. Pat. No. 5,225,820, to Clerc. By way of example, a suitable material for use as conductive film 12 may be indium-tin-oxide (ITO), which is substantially optically transparent and electrically conductive. Anode plate 1 also comprises a cathodoluminescent phosphor coating 3, deposited over conductive film 12 so as to be directly facing and immediately adjacent gate electrode 6. In the Clerc patent, the conductive bands of each series are covered with a particulate phosphor coating which luminesces in one of the three primary colors, red, blue and green, labeled 3.sub.R, 3.sub.B, 3.sub.G respectfully.
Selected groupings of microtip emitters 5 of the above-described structure are energized by applying a negative potential to cathode electrode 9 relative to the gate electrode 6, via voltage supply 15, thereby inducing an electric field which draws electrons from the apexes of microtips 5. The potential between cathode electrode 9 and gate electrode 6 is approximately 70-100 volts. The emitted electrons are accelerated toward the anode plate 1 which is positively biased by the application of a substantially larger positive voltage from voltage supply 11 coupled between the cathode electrode 9 and conductive film 12 functioning as the anode electrode. The potential between cathode electrode 9 and anode electrode 12 is approximately 300-1000 volts. Energy from the electrons attracted to the anode conductive film 12 is transferred to particles of the phosphor coating 3, resulting in luminescence. The electron charge is transferred from phosphor coating 3 to conductive film 12, completing the electrical circuit to voltage supply 11. Charge can also be transferred by secondary electron emission. The image created by the phosphor stripes is observed from the anode side which is opposite to the phosphor excitation, as indicated in FIG. 1.
The process of producing each frame of a display using a typical trichromatic field emission display includes (1) applying an accelerating potential to the red anode stripes while sequentially addressing the gate electrodes (row lines) with the corresponding red video data for that frame applied to the cathode electrodes (column lines); (2) switching the accelerating potential to the green anode stripes while sequentially addressing the rows lines for a second time with the corresponding green video data for that frame applied to the column lines; and (3) switching the accelerating potential to the blue anode stripes while sequentially addressing the row lines for a third time with the corresponding blue video data for that frame applied to the column lines. This process is repeated for each display frame.
It is to be noted and understood that true scaling information is not intended to be conveyed by the relative sizes and positioning of the elements of anode plate 1 and the elements of emitter plate 2 as depicted in FIG. 1. For example, in a typical FED shown in FIG. 1 there are approximately one hundred arrays 4, of microtips per display pixel, and there are three color stripes 3.sub.R, 3.sub.B, 3.sub.G per display pixel. Furthermore, phosphor coating 3 may not be a dense coating, but instead be comprised of an arrangement of phosphor particles which have adhered to conductors 12.
The conventional process for forming the microtips in the emitter plate of the flat panel display is taught by the Spindt et al. ('704) patent. This process involves forming a sacrificial layer, called a lift-off layer, on the surface of the gate using low angle evaporation techniques well known in the industry. The lift-off layer is illustratively nickel. The microtips are formed by evaporation, at a normal angle, of the tip metal into the holes formed in the gate metal and underlying insulator material. The tip metal is illustratively molybdenum. The superfluous tip metal located on top of the lift-off layer, and the lift-off layer are then dissolved by an electrochemical process which then exposes the gate metal and the microtips.
Many techniques have been proposed for enhancing microtip emission efficiency. Such techniques include 1) interferometric lithography, as described in Journal of Vacuum Science & Technology B, Bozler, Carl O., Harris, Christopher T., Rabe, Steven, Rathman, Dennis D., Hollis, Mark A., and Smith, Henry I., "Arrays of gated field-emitter cones having 0.32 .mu.m tip-to-tip spacing," pp.629-632, Volume 12, Number 2, March/April 1994; 2) application of tip surface coatings, as described in Journal of Vacuum Science & Technology B, Zhirnov, V. V., and Givargizov, E. I., "Chemical vapor deposition and plasma-enhanced chemical vapor deposition carbonization of silicon microtips," pp.633-637, Volume 12, Number 2, March/April 1994; and 3) changing the shape of the electron emitter surface, as described in Journal of Vacuum Science & Technology B, Lee, Bo, Elliott, T. S., Mazumdar, T. K., McIntyre, P. M., Pang, Y., and Trost, H. J., "Knife-edge thin film field emission cathodes on (110) silicon wafers," pp.644-647, Volume 12, Number 2, March/April 1994, and also described in Journal of Vacuum Science & Technology B, Pogemiller, J. E. Busta, H. H., and Zimmerman, B. J., "Gated chromium volcano emitters," pp.680-684, Volume 12, Number 2, March/April 1994, all incorporated herein by reference.
It is desirable to achieve the highest possible emission efficiency for field emission displays through enhancing microtip emission efficiency. However, there is a maximum microtip emission efficiency which can be achieved for each of the fabrication techniques taught in the articles listed above. There exists a need for a manufacturing technique which further increases the emission efficiency of any emission structure after its initial fabrication by creating microtips with radii smaller than current fabrication limits.